The invention relates to a process for thermally treating a flowable material in a fluidised bed. It has particular application to the drying or calcining of a granular, compacted or granulated substance using heated contact surfaces in a fluidised bed.
To such processes, the material to be treated comes into contact with heated contact surfaces, thus transmitting heat to the particles of material. The contact surfaces are incorporated in the layer of material which is to be treated. The introduction of gas causes movement of the layer in order to intensify the heat transfer. A known apparatus for carrying out such treatment is a fluidised bed contact drier, for example. It is known to use hot air for the gas treatment of the fluidised layer. However, this process imposes limits on the temperatures which can prevail in the layer. Temperatures in the fluidised bed which are above the burning point of the material which is to be treated should not be permitted, as the material would start to burn. This means that he temperatures are kept relatively low so as not to damage the product, with the result that the process to be carried out lasts longer than might be the case if higher temperatures were used.
In other cases, it can be desirable to use higher temperatures for the purpose of sterilizing of disinfecting the product by treating it at a correspondingly high temperature. This is the case, for example, with thermal treatment, particularly when drying sewage sludge, wherein the product has to be hygienically satisfactory so that it can be used, for example, in agriculture for improving the soil. In each of these processes, there is the problem of disposal of the gases discharged from the fluidised bed.